Summary: Clinical trials using porcine tissues or cells are underway. Since all pigs carry in their genome porcine endogenous retroviruses (PERV), these xenotransplantation procedures carry the risk of exposure to and transmission of PERV. This year, we analyzed whether primary cells and cell lines derived from non-human primates of several different species (rhesus macaque, baboon, and African green monkey) were permissive to PERV replication. We performed this analysis to determine whether non-human primates are a suitable model for assessing in vivo infectivity and potential for pathogenicity of PERV. In all cases, we found that the virus could enter the cells, become integrated, and express viral RNA, but the viral infection did not spread in the NHP cultures. Further analyses demonstrated that the block to permissive replication may occur at several different stages, including inefficient viral entry and viral assembly (A. Ritzhaupt, et al, J. Virol., In Press). The analysis demonstrated that NHP do not likely serve as a permissive model for PERV replication in vivo, and therefore, data from such studies should be interpreted with caution. As a component of this project, we developed a sensitive assay for detection and quantitation of PERV RNA and DNA that can be used to analyze samples from cell culture experiments, animal tissues, and human clinical samples (Argaw, et al, J. Virol. Methods, In Press). The second project we completed involved an analysis of PERV derived from pig cells that were engineered to express one of the human complement regulatory proteins, CD59 (Takefman, et al, J. Virol. 75:4551). The aim of this study was to reproduce what would likely happen in pigs transgenic for this molecule. It has been proposed that PERV expressed from pigs that are transgenic for human CRPs may be resistant to the virolysis and neutralizing activity of complement-mediated protection. We found that the human CD59 protein is incorporated into virions derived from pig cells expressing hCD59, and that the presence of this molecule inhibited complement-mediated lysis, but not neutralization of infectivity of PERV. Therefore, one component of complement-mediated protection against PERV infection is still retained.